Biologists have known the ocean has a massive nitrogen-fixing system, but they haven't been able to identify all the sources. Jonathan Zehr at the University of California in Santa Cruz explains that "in the open ocean, there are only one or two organisms known to fix nitrogen."

Now, however, he and colleagues at several other institutions have identified a third: a type of photosynthetic bacteria. In reporting it today in the journal Nature, they say there likely are many other unknown nitrogen-fixing microbes in the sea. This photosynthetic one is the first they have been able to cultivate in the laboratory and put through its nitrogen-fixing paces.

The discovery is as important for climate scientists as it is for marine biologists. The ocean's ability to remove atmospheric carbon dioxide has been poorly known, because the biological cycle involved hasn't been fully explained.

What is known is that the plankton's photosynthetic plants break down carbon dioxide and lock up the carbon in their shells. These eventually dissolve in seawater or drift to the bottom and become encased in sediments.

Dr. Zehr explains that scientists need to know more about the production of the nutrients driving this carbon-dioxide removal process, if they are to construct realistic computer models of possible global warming.

"We've identified another nutrient source," he says. It's a big one. He estimates that "there's potentially as much biomass of these bacteria as of the other two known nitrogen-fixers in the open ocean."

But he and his colleagues suspect more are to be found. They may not be photosynthetic. But they still could contribute significant amounts of nitrogen fertilizer.

Instead of trying to find suspicious-looking microbes, Zehr goes after a telltale genetic fingerprint: the presence of the protein responsible for nitrogen fixation. This, in turn, indicates the presence of a microbe with the nitrogen-fixing gene.

That's what the team found in samples gathered off Hawaii by David Karl of the University of Hawaii in Honolulu. Given that clue, John Waterbury at the Woods Hole Oceanographic Institution in Massachusetts was able to isolate the organism. It appears to belong to the grouping cyanobacteria, also known as photosynthetic blue-green algae.

Commenting on the discovery in Nature, Jed Fuhrman and Douglas Capone with the University of Southern California in Los Angeles observe that "the closer we look at the oceans, the more important the tiniest organisms appear to be."